Conpherence - only validate new participants when validating TYPE_PARTICIPANT transactions

Summary: Fixes T7693.

Test Plan: made a conpherence with my main test account and a throwaway test account. deleted the test account. removed myself from the thread. added myself back to the thread.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7693

Differential Revision: https://secure.phabricator.com/D12306